My lab previously identified H3K4 methyltransferases MLL3 and MLL4 (Cho YW, JBC 2007) and H3K27 demethylases UTX and JMJD3 (Hong S, PNAS 2007). Using a nuclear protein PTIP as the bait, we isolated from cell nuclei a protein complex that contains H3K4 methyltransferases MLL3/MLL4, H3K27 demethylase UTX, PTIP and a novel protein PA1 (Cho YW, JBC 2007). Further, we show that PTIP is required for PPAR&#947; and C/EBP&#945; expression and adipogenesis (Cho, YW, Cell Metab 2009). To understand the physiological roles of MLL3/MLL4 and associated factors, we have knocked out MLL3, MLL4, UTX and PA1 in mice. Because of their embryonic lethality, we have generated conditional knockout (KO) of MLL4, UTX and PA1. By crossing these conditional KO with Myf5-Cre mice, we found that MLL4, PTIP and PA1 are essential for adipogenesis while UTX is dispensable. To investigate UTX function, we turned to mouse embryonic stem (ES) cells. We found that UTX controls ES cell differentiation and early embryonic development independent of H3K27 demethylase activity (Wang C, PNAS 2012). To understand the biological role of the H3K27 demethylase activity of UTX, we have generated enzyme-dead UTX knockin mice. We recently identify MLL3/MLL4 as H3K4 mono- and di-methyltransferases that are essential for enhancer activation during cell differentiation (Lee JE, eLife 2013). Enhancers play a central role in cell-type-specific gene expression and are marked by H3K4me1/2. Active enhancers are further marked by H3K27ac. However, the methyltransferases responsible for H3K4me1/2 on enhancers remain elusive. Furthermore, how these enzymes function on enhancers to regulate cell-type-specific gene expression is unclear. We identify MLL4 as a major mammalian H3K4 mono- and di-methyltransferase with partial functional redundancy with MLL3. Using adipogenesis and myogenesis as model systems, we show that MLL4 exhibits cell-type- and differentiation-stage-specific genomic binding and is predominantly localized on enhancers. MLL4 co-localizes with lineage-determining transcription factors (TFs) on active enhancers during differentiation. Deletion of MLL4 markedly decreases H3K4me1/2, H3K27ac, Mediator and Polymerase II (Pol II) levels on enhancers and leads to severe defects in cell-type-specific gene expression and cell differentiation. Together, these findings identify MLL3/MLL4 as major mammalian H3K4 mono- and di-methyltransferases essential for enhancer activation during cell differentiation.